This invention relates to degasification of coal seams, and more particularly to a procedure for producing and maintaining a generally horizontal gas drainage borehole in a coal seam.
Underground coal mines must always deal with methane gas, which permeates the coal, in order to avoid explosions. The most common way of avoiding an explosive concentration of methane has historically been to dilute the methane with excess air, as by ventilation. More recently various procedures have been both proposed and implemented for draining a significant portion of the methane from the coal seam prior to mining of the seam. In certain instances the methane drainage operation can be so carried out as to result in collection of a useful gaseous methane fuel, in addition to enhancing the safety of any subsequent mining of the drained area.
One approach to methane drainage involves drilling a well from the earth's surface into the coal seam. In U.S. Pat. No. 3,384,416, the well bore is then used to carry out a hydraulic fracturing and propping of the coal seam. U.S. Pat. No. 4,043,395 teaches passing a carbon dioxide-containing fluid down the well bore into contact with the coal, and subsequently recovering the carbon dioxide plus desorbed methane. U.S. Pat. No. 4,245,699 discloses using the well bore to form fissures within the coal seam, as by pressuring and rapid depressuring.
Another approach to methane drainage involves penetrating the coal seam from an underground working face. U.S. Pat. No. 3,650,564 teaches drilling a plurality of parallel holes into the coal seam and then fracturing the seam as by activating explosives in some but not all of the holes, and drawing off methane from the remaining holes. U.S. Pat. No. 4,303,274 discloses degassing a mineable coal seam by drilling horizontal holes into an underlying or overlying seam, and producing gas from the boreholes, such that migration of gas into the mineable seam is reduced. Finally, U.S. Pat. No. 4,321,967 discloses a plastic suction probe which is sealed or cemented into a borehole to allow vacuum pumping of methane therefrom.